Conventionally, a method has been disclosed wherein a water-soluble binder, such as starch, is used as a binder for molding. By that method aggregate and the binder are agitated so that the blended aggregate foams, to thereby form a mold. Since starch or the like is used, the advantages obtained by that method are that substantially no unpleasant odor or gas is generated while molding or while pouring molten metal into the mold and that the binding power of the binder is weakened by means of heat while casting takes place, so that the aggregate can be easily removed from a cast (for example, see Patent Literature 1).
A method for molding has been also disclosed wherein by using liquid glass, i.e., sodium silicate, as the water-soluble binder, generation of any unpleasant odor or gas is further reduced (for example, see Patent Literature 2).
By the method for casting that uses the blended aggregate that is foamed after the aggregate and the water-soluble binder are agitated, the mold may be weakened by absorbing moisture. The higher the humidity around the mold, the more the strength of it decreases. If the mold of which the strength decreases were used for casting, it, or a part of it, would break during the transportation of it or before the molten metal that has been poured solidifies. Thus, the resistance to moisture of the mold has been required to be increased.
The present invention aims to provide a binder composition for molding and blended aggregate for molding, by which a mold that has an excellent resistance to moisture can be obtained. It also aims to provide a mold that has an excellent resistance to moisture.